1. Field of the Invention
The present invention relates to a method of making a nanocomposite magnet powder of an iron-based rare-earth alloy by an atomization process.
2. Description of the Related Art
Nd—Fe—B based iron-based rare-earth magnet alloys are currently used extensively in sintered magnets and bonded magnets. These magnets are produced in different ways. More specifically, a sintered magnet is made by pulverizing a magnet alloy, obtained by an ingot casting process, a strip casting process or any other process, compacting the pulverized powder, and then sintering the powder compact. On the other hand, a bonded magnet is made by rapidly cooling and solidifying a molten alloy by a melt spinning process, for example, pulverizing the resultant rapidly solidified alloy into a powder, compounding the powder with a resin, and then molding the mixture into a desired shape. In this manner, no matter whether the magnet to be produced is a sintered magnet or a bonded magnet, its magnet powder is always obtained by pulverizing the material magnet alloy. That is to say, the manufacturing process of magnets normally includes a pulverizing process step as an indispensable process step.
However, there are some methods of making a magnet powder without performing such a pulverizing process step. A gas atomization process is one of such methods that are known in the art. In the gas atomization process, a melt of an alloy is sprayed through a nozzle, for example, into an inert gas, and is made to collide against the gas, thereby cooling the melt droplets. In this manner, spherical particles with particle sizes on the order of several tens of micrometers can be formed directly. In the gas atomization process, the droplets of the molten metal are solidified while being carried in the gas-flow, thus forming substantially spherical particles. The powder obtained by the gas atomization process (i.e., an atomized powder) has preferred shapes and particle sizes as a magnetic powder to make a bonded magnet.
If the atomization process is used to produce a bonded magnet, then the atomized powder can be used as it is as a magnet powder for a bonded magnet. Thus, no mechanical pulverizing process step is needed and the manufacturing cost can be reduced significantly. It should be noted, however, that the particle sizes of such an atomized powder are greater than those of a magnet powder to make a sintered magnet. Accordingly, it is difficult to use the atomized powder as it is as a magnet powder for a sintered magnet.
In a rapidly solidified rare-earth alloy magnet powder, which is currently used extensively to make a bonded magnet, an Nd2Fe14B based compound phase with a crystal grain size of about 20 nm to about 200 nm is finely dispersed in the powder particles. Such a nanocrystalline structure is formed by rapidly cooling a molten alloy with a particular composition by a melt spinning process, for example, to make an amorphous alloy thin strip and then thermally treating and crystallizing the amorphous alloy thin strip.
Meanwhile, high-performance magnets, having a quite different metal structure from that of the rapidly solidified magnet described above, are also under vigorous development. A typical example of those magnets is a composite magnet called a “nanocomposite magnet (exchange spring magnet)”. The nanocomposite magnet has a metal structure in which hard and soft magnetic phases are finely dispersed and in which the respective constituent phases are magnetically coupled together through exchange interactions. The respective constituent phases of the nanocomposite magnet have nanometer-scale sizes and its nanocrystalline structure, defined by the sizes and dispersiveness of the respective constituent phases, has significant effects on its magnet performance.
Among those nanocomposite magnets, a magnet in which an Nd2Fe14B based compound phase (i.e., hard magnetic phase) and α-Fe, iron-based boride and other soft magnetic phases are distributed in the same metal structure, attracts particularly much attention. In the prior art, such a nanocomposite magnet is also made by rapidly cooling a molten alloy by the melt spinning process and then thermally treating and crystallizing the rapidly solidified alloy. If the powder of such a rapidly solidified magnet could be made by the gas atomization process, then no other pulverizing process step would be needed and the manufacturing cost could be reduced significantly.
Actually, however, it is very difficult to make the rapidly solidified magnet powder by the atomization process. This is because the melt-quenching rate by the atomization process is lower than that by the melt spinning process by as much as one to two orders of magnitude. Thus, a sufficiently amorphized alloy structure cannot be formed by the conventional atomization process.
As for non-nanocomposite rapidly solidified magnets (i.e., rapidly solidified magnets including only the Nd2Fe14B based compound phase), a method of making an amorphous alloy producible even by the atomization process with such a low quenching rate by increasing the amorphous forming ability (i.e., quenchability) of the alloy with the addition of TiC, for example, was proposed.
However, as for an α-Fe/R2Fe14B based nanocomposite magnet, it is difficult to produce an actually usable, high-performance magnet by the atomization process. The reason is that if the quenching rate is as low as in the atomization process, the soft magnetic α-Fe phase easily nucleates and grows earlier than the R2Fe14B phase and increases its size so much that the exchange interactions among the respective constituent phases weaken and that the magnetic properties of the resultant nanocomposite magnet deteriorate significantly.
In order to overcome the problems described above, a primary object of the present invention is to make a powder of a nanocomposite magnet with excellent magnetic properties producible by the atomization process.